This invention relates generally to coaxial cable isolators, and specifically to an antenna coaxial cable ground isolator for use in audio-video entertainment systems.
When a cable TV or other externally grounded coaxial antenna cable is connected to an extensive audio-video entertainment system, it is common to encounter problems caused by circulating ground currents. These currents are due to the difference in line frequency AC potential between the system audio ground reference provided by the AC wall outlet and the ground provided by the coaxial antenna cable. The resulting ground loop current, flowing through the system wiring, produces hum and noise voltages which are amplified and reproduced by the system's speakers. In severe cases, the signal may be visible on a television monitor as a slowly rising horizontal line.
Heretofore, audio-video system installers have attempted to alleviate this problem by several methods. The most common of these has been to isolate the system equipment from the ac supply ground by means of "3 prong to 2 prong" wall socket adapters. This practice, although sometimes effective, presents a serious shock hazard and is contrary to safety and electrical codes.
Another technique has been to connect the cable antenna line through "back to back" 75 ohm to 300 ohm "balun" (balanced to unbalanced) transformers incorporating DC isolation. Although successful in breaking the ground loop, this method typically causes signal degradation due to losses and stray signal pickup.
Unfortunately, the optimum solution to the problem, use of a coaxial cable capacitive isolator for providing an interruption in the DC and low frequency continuity of a cable is not widespread in the audio-video installation field, due mainly to the lack of commercially viable products.
The theory and application of capacitive isolators is well documented by several prior art references. U.S. Pat. No. 3,393,384 to Bailey et al. provides a basis for such a coaxial isolator. U.S. Pat. No. 3,908,176 to De Boer carries its development further, describing an isolator specifically designed for cable TV antenna systems. It must be noted, however, that both these and subsequent references relate to isolators constructed coaxially with cylindrical capacitive elements. Although providing excellent high frequency coupling and minimum VSWR (voltage standing wave ratio), this physical configuration is both difficult and costly to manufacture. The capacitors necessary for such an isolator are not standard commercially available components and must be specifically designed and fablricated for the application.
Heretofore, the inexpensive alternative to the coaxial isolator described above has been to connect discrete wire leaded capacitors in series with both the shield and inner conductors of the coaxial cable requiring isolation. This approach presents serious problems, however, in that the lead inductance and equivalent series resistance of the discrete components tend to reduce performance, especially at UHF frequencies. Specifically, cables employing such isolation techniques are subject to interference from external electromagnetic fields. Such interactions are due to the parasitic impendance placed in series with the coaxial shield, allowing interfering signal voltages induced on the shield conductor to couple with those of the desired signal internal to the cable. In addition, the discontinuity resulting from the interruption in the transmission line evironment produces reflections, a high VSWR, and additional signal loss.
The principal object of the present invention is to permit manufacture by modern automated electronic assembly techniques, a cost-effective isolator capable of the radio frequency performance of the cylindrical capacitor variety.
Another object of the invention is to provide for the dissipation of accumulated static charges on the coaxial line.
Still another object of the invention is to provide protection for associated electronic equipment connected to the output of the device from high voltage transients which may be produced by nearby lightning strikes.
Further objects and advantages of the invention will become apparent from a consideration of the ensuing drawings and detailed description.